Conventionally, printed wiring boards have been used in consumer use electronic products such as home use electronic products, or industrial use electronic equipments such as computer or the like, by providing a conductor layer on a surface of an insulating base material, followed by forming the conductor layer into a circuit shape by etching or the like, and mounting devices such as IC chips or capacitors on the circuit.
In the printed wiring board, on a surface of an insulating base material, a copper foil is bonded as disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 07-007272) or a copper layer is formed by additive method as disclosed in Patent Document 2 (Japanese Patent Laid-Open No. 07-111386) or by employing sputtering deposition as disclosed in Patent Document 3 (Japanese Patent Laid-Open No. 2000-286530). After manufacturing a copper-clad laminate, an etching resist layer is provided on the formed copper layer, then an etching pattern is exposed and developed, and then copper etching is performed to form the circuit shape.
For the circuits manufactured in these ways, fine pitch has been developed in accordance with requirement of miniaturization for the circuits. However, based on the purpose of using the circuits, the circuits can be broadly categorized as a signal transmission circuit having narrow width for controlling drive systems and a circuit for obtaining power supply or GND (hereinafter simply referred to as a “power supply circuit or the like”, meaning that power supply circuit and GND circuit are included). The signal transmission circuit is a circuit for transmitting signal current for controlling ON/OFF operation, calculating speed, or the like, in which large current does not flow in general. On the other hand, the power supply circuit or the like serves to supply current for power supply to chip devices, capacitors, or the like mounted on the printed wiring board, or serves as GND. Therefore, the power supply circuit or the like is a circuit in which substantially large current flows, in comparison to the above described signal current.
In addition, electrical resistance R(Ω) of the circuit is calculated by a calculation formula of R(Ω)=ρ×L/S where ρ is specific resistance of the material (Ω×cm), L is length (cm), and S is cross-sectional area (cm2). Further, power consumption amount W (Watt) is calculated by a calculation formula of W(Watt)=I2R where I(A) is current. As can be seen from these formulas, as the area reduces, resistance value of the circuit increases. Further, as resistance value increases, power consumption amount increases and consequently heat generation amount in operation increases. Therefore, a design is employed in which circuit width of the power supply circuit or the like is increased in comparison to circuit width of the signal transmission circuit, in order to reduce heat generation amount from the printed wiring board in operation.
On the other hand, in recent years, reduction of weight, size and thickness has been continuously required on the devices such as electronic equipments incorporating the printed wiring board. Accordingly, of course, reduction of size and the thickness of the printed wiring board has been also required, corresponding to the limited accommodating space of the devices.    [Patent Document 1] Japanese Patent Laid-Open No. H07-007272    [Patent Document 2] Japanese Patent Laid-Open No. H07-111386.    [Patent Document 3] Japanese Patent Laid-Open No. 2000-286530
However, in the conventional method for manufacturing the printed wiring board, the design concept for the signal transmission circuit and for the power supply circuit or the like can not be changed. Thus, if it is attempted that the signal transmission circuit and the power supply circuit or the like are together formed in a same reference plane, it is inevitable to form the power supply circuit or the like having larger width than that of the signal transmission circuit, which also inhibits miniaturization of the printed wiring board.
Further, for example, in the case of a substrate formed under assumption that large current flows therein, the power supply circuit or the like is formed by etching a layer formed from thick copper, while the signal transmission circuit is formed with copper thickness which matches circuit characteristics. Therefore, it is required to separately construct the layers, which also inhibits miniaturization of the printed wiring board.
Indeed, it is considerable to reduce supply voltage or use a material having superior electrical conductivity than copper which forms the circuit, when it is attempted that width of the power supply circuit or the like is reduced without increasing heat generation amount from the printed wiring board. However, these approaches are not practical.
In view of the above described problems, a printed wiring board and a method for manufacturing the printed wiring board are required in which circuit widths of the signal transmission circuit and the power supply circuit or the like, which conventionally require to have greatly different circuit widths, are close to each other as much as possible and substantial miniaturization can be achieved.